Method of forming a superconductor

ABSTRACT

Disclosed herein is a method of forming a superconductor, comprising the steps of: providing a substrate and exposing the substrate to a first atmosphere, including precursors to form a first epitaxial layer segment. The first layer segment is then exposed to a second atmosphere, including precursors to form a second epitaxial layer segment, and the second layer segment is exposed to a third atmosphere including precursors to form a third epitaxial layer segment. Each of the first and third layer segments are each formed from a superconductor material and the second layer segment is formed from a material different from the first and third layer segments and the first, second and third layer segments have a collective thickness, the third layer segment having an outer surface with a roughness which is less than that of a single layer of the superconductor material with a thickness equal to the collective thickness.

REFERENCE TO CO-PENDING APPLICATION

The subject matter of U.S. application Ser. No. 08/925,887 filed Sep. 8,1997 entitled A METHOD OF FORMING A SUPERCONDUCTOR is incorporatedherein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to superconductor materials.

2. Description of the Related Art

Superconductor materials are gaining ever increasing attention for theirability to carry significantly large currents without resistance. Evenat high frequencies well into the microwave regime and at large currentlevels, these materials can exhibit negligible dissipation. Theso-called high temperature superconductors are especially important formany applications because they can exhibit such properties attemperatures of 77 K or higher. One promising application for thesematerials is in the form of epitaxially grown superconductor this filmsfor use in wireless communication systems, both satellite and groundbased.

The high temperature superconductors are generally anisotropic oxidematerials. In a crystal of a typical high temperature superconductor,say YBa₂Cu₃O₇₋₈, currents are readily carried in the ‘a’ or ‘b’crystallographic directions while the ‘c’ direction can only sustain asmall current without significant dissipation. Many other hightemperature superconductor materials are even more anisotropic. As aresult, optimal current-carrying capacity in a film requires anorientation of the ‘c’ axis everywhere perpendicular to the substrate.This geometry enables the current to flow in the ‘a’ and ‘b’ directionsonly. Even with the proper alignment of the ‘c’ axis, the alignment ofthe ‘a’ and ‘b’ directions is relevant for the current-carryingcapacity. In some applications, it may be preferable for the ‘a’ and ‘b’directions to be consistent throughout the film, although forYBa₂CU₃O₇₋₈, and many other high temperature superconductors, this maylead to different properties in the ‘a’ and ‘b’ directions. Largecurrents can also be carried if the ‘a’ and ‘b’ directions occasionallyinterchange via a mechanism called “twinning”. It is generallyconsidered not desirable to have other relative orientations of the ‘a’and ‘b’ directions in different parts of the film since these lead tolarge angle grain boundaries which are found to decrease the currentcarrying capacity of the film.

Typically, as these films are grown their outer surface tends toroughen. This can be due to particulates attaching to the film duringgrowth or the nucleation of undesired orientations. Even if suchdifficulties are avoided the surface will tend to roughen as it growsand can be characterized by a series of peaks and valleys. This isusually attributed to a “spiral growth mode” known to be typical forthese materials. In such films, regardless of the height of the peaks,the current is limited by the thickness in the valleys. Further, anycurrent carried or induced near the surface of a peak must necessarilytravel in the ‘c’ axis direction to pass through a valley.

Moreover, attempts to continue the growth process and increase theuseful thickness have progressively diminishing returns thereon sincethe peaks tend to gain height at the expense of the valleys. In otherwords, the valleys do not see a commensurate increase in height.

In addition to reducing the current carrying capacity, rough films haveother undesirable properties. These include increased microwave surfaceresistance and increased electrical noise. Such defects in the filmswill also make patterning the film difficult and hamper the filmdevelopment of more complicated multi-layer structures on top of thesuperconducting film.

It is an object of the present invention to provide novelsuperconductors.

SUMMARY OF THE INVENTION

Briefly stated, the invention involves a method of forming asuperconductor, comprising the steps of:

providing a substrate;

exposing the substrate to a first atmosphere, including precursors toform a first epitaxial layer segment,

exposing the first layer segment to a second atmosphere, includingprecursors to form a second epitaxial layer segment, and

exposing the second layer segment to a third atmosphere includingprecursors to form a third epitaxial layer segment,

wherein each of the first and third layer segments are each formed froma superconductor material and the second layer segment is formed from amaterial different from the first and third layer segments,

wherein the first, second and third layer segments have a collectivethickness, the third layer segment having an outer surface with aroughness which is less than that of a single layer of thesuperconductor material with a thickness equal to the collectivethickness.

In another embodiment, there is provided a composite superconductor filmapplied to a substrate, the film having a thickness of at least 5000Angstroms and an outer surface having an average roughness not exceeding250 Angstroms.

In another aspect of the present invention, there is provided a layer ofsuperconductor materials, the layer having a current carrying capacityand an inner discontinuous epitaxial region formed in the presence ofdielectric precursor materials and at a concentration so as not tosubstantially reduce the current carrying capacity.

BRIEF DESCRIPTION OF THE DRAWINGS

Several preferred embodiments of the present invention will now bedescribed, by way of example only, with reference to the appendeddrawings in which:

FIG. 1 is an Atomic Force Microscope (hereinafter referred to as “AFM”)image of a superconductor sample (5000 Å);

FIG A is a schematic view of a pair of superconductors.

FIG. 2 is an AFM image of another sample (5000 Å);

FIG. 3 is an AFM profile of the sample shown in FIG. 1 (5000 Å);

FIG. 4 is an AFM profile of the sample shown in FIG. 2 (5000 Å);

FIG. 5 is an AFM profile of still another superconductor sample (8000Å);

FIG. 6 is an AFM profile of yet another sample (8000 Å); and

FIG. 7 is a comparative plot of critical current density versustemperature for the samples illustrated in FIGS. 5 and 6.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

As will be described herein below, there is provided a method of forminga superconductor, comprising the steps of:

providing a substrate;

exposing the substrate to a first atmosphere, including precursors toform a first epitaxial layer segment,

exposing the first layer segment to a second atmosphere, includingprecursors to form a second epitaxial layer segment, and

exposing the second layer segment to a third atmosphere includingprecursors to form a third epitaxial layer segment,

wherein each of the first and third layer segments are each formed froma superconductor material and the second layer segment is formed from amaterial different from the first and third layer segments,

wherein the first, second and third layer segments have a collectivethickness, the third layer segment having an outer surface with aroughness which is less than that of a single layer of thesuperconductor material with a thickness equal to the collectivethickness.

In another embodiment, there is provided a composite superconductor filmapplied to a substrate, the thin film having a thickness of at least5000 Angstroms and an outer surface having an average roughness notexceeding 250 Angstroms.

The term ‘epitaxial’ is intended to mean that the position of the atomsin each layer is substantially determined by the position of the atomsin the preceding layer. This does not preclude the possibility ofoccasional defects such as vacancies, pinholes, twin boundaries anddislocations that are known to occur in such systems.

The term ‘layer segment’ is used herein to refer to the fact that,though formed expitaxially, the individual layer segments may, forexample, present themselves both optically and electrically as a singlecrystal thin film and therefore may have no substantially discernablefeatures to set them apart.

The term ‘film’ is intended to include those generally referred to as‘thick films’ and ‘think films’, the latter whose thicknesses usually donot exceed 5000 Angstroms.

Preferably, the second layer segment is discontinuous. In other words,the second layer segment may take the form of islands on the first layersegment, or instead have pinholes or inclusions. The first and thirdlayer segments are formed from either the same or different oxidesuperconductor material and the first, second and third layer segmentshave a collective current density which is substantially equal to thecurrent density of the first layer segment. More preferably, the oxidesuperconductor material is a high temperature superconductor, and stillmore preferably a copper-oxide superconductor.

The second layer segment may be formed from an oxide material, includingan insulator material or a superconductor material. Preferably, theinsulator material is a dielectric material selected from a groupcomprising SrTiO₃, LaGaO₃, PrGaO₃, NdGaO₃, SrLaGaO₄, CeO₂, LaAlO₃,LaSrAlO₄. More particularly, the dielectric material isBa_(x)Sr_(1-x)TiO₃ (hereinafter referred to as ‘BSTO’).

Preferably, the superconductor material is selected from the groupcomprising RBa₂CU₃O₇₋₈ wherein R is a rare earth, or a Tl-, Pb-, Bi- orHg-based copper-oxide superconductor materials, such as for exampleY-Ba-Cu-O, Bi-Sr-Ca-Cu-O, Tl-Ba-Ca-Cu-O. More particularly, thecopper-oxide superconductor is YBa₂Cu₃O₇₋₈ (hereinafter referred to as‘YBCO’).

The second layer segment may or may not cover the entire outer surfaceof the first layer segment. The surface of the second layer segment mayor may not be smoother than the outer surface of the first layersegment. This will depend on the extent of growth of both layersegments.

In one embodiment, the first and third layer segments have a cumulativethickness and the current-carrying capacity of this cumulative thicknessshould be greater that the current-carrying capacity of the first layersegment and roughly proportional to the cumulative thickness. In otherwords, the critical current density for the first layer segment and forthe cumulative thickness of the first, second and third layer segmentsshould be substantially equal.

If desired, the growth of the third layer segment may be continued untilits outer surface becomes rough. The third layer segment may then bere-exposed to a fourth atmosphere to form a fourth layer segment, and soon to provide films with arbitrary cumulative thicknesses.

In one embodiment, a first superconductor layer segment is epitaxiallygrown on a substrate. During this growth, the surface of the firstsuperconductor layer segment 10 becomes relatively rough and forms apattern of peaks and valleys. At some stage in the process, furthergrowth becomes non-beneficial because the peaks become still higherwithout corresponding growth in the valleys. In this case, the firstsuperconductor layer segment has a current carrying capacity which canbe defined as some function of the current density as well as the crosssectional area of the current channel, that is the channel between thesubstrate and the ‘lowest’ valley.

It has been found that additional epitaxial growth can be carried out onthe surface of the first superconductor layer segment under certainconditions so as to, in effect, ‘fill in’ the valleys thereon. In otherwords, the conditions of further epitaxial growth are selected in such amanner that the epitaxial growth occurs at a greater rate in the valleysthan on the peaks. In one preferred embodiment, a second layer segment14 of dielectric material is eppitaxially grown on the surface of thefirst superconductor layer segment. The presence of this dielectriclayer segment is found to influence the growth of a third superconductorlayer segment such that the outer surface of the third layer segment 16is smoother that the outer surface of the first superconductor layersegment. Moreover, the surface of the third superconductor layer segmentis also found to be of higher quality while the second superconductorlayer segment is capable of carrying current densities equivalent to thefirst layer segment.

In this case, the surface of the third superconductive layer segment 16has a roughness r₁ which is less than the roughness r₂ of a single layerof the superconductor material, as shown at 18, with the same thickness“t”.

In some applications, it may be preferred to use relatively thickdielectric layer segments, such that the first and third superconductorlayer segments are isolated from each other by a continuous seconddielectric layer segment. However, in some instances, it is in factdesirable to have physical and electrical connections between the firstand third superconductor layer segments. In one preferred embodiment,this connection may arise due to particulates in the firstsuperconductor layer segment or pinholes in the second dielectric layersegment. In the latter case, the third superconductor layer segment isthen deposited on the second dielectric layer segment in most places aswell as in the pinholes, thereby making the direct connection to thefirst superconductor layer segment. These ‘filled-in’ pinholes have theeffect of shorting out the adjacent superconductor layer segmentsthrough the second insulating dielectric layer segment, creating asituation in which the first and second superconducting layer segmentsin the composite material are thus electrically connected.

In another preferred embodiment, the second layer segment is grown to bethin enough and under suitable conditions to form ‘islands’ ofdielectric material on the first superconductor layer segment. The thirdsuperconductor layer segment is then grown epitaxially on the secondlayer segment, resulting in a single composite superconducting film,that is with dielectric interstices embedded within it. Moreover, theprocess of introducing dielectric material between the superconductorlayer segments can be repeated many times, to increase the thickness ofthe resulting superconducting film still further.

In yet another preferred embodiment, the second layer is of anelectrically conducting oxide material. This conducting oxide may or maynot be a high temperature superconductor. A second superconductor layersegment provides a significantly improved electrical continuity betweenthe first and third superconductor layer segments provided the ambienttemperature is below the critical temperature of all the superconductormaterials in the film structure.

If desired, the first, second and third layer segments may be arrangedto form together a signal crystal layer, with a single current carryingchannel. Remarkably, this provides a substantial increase in currentcarrying capacity and apparently the presence of the resultingdielectric interstices do not seem to impair the current density for thematerial.

In one exemplified embodiment, a first layer segment of YBCO isepitaxially grown on LaAlO₃. A BSTO second layer segment is grown on theYBCO first layer segment and a YBCO third layer segment is then grown onthe BSTO second layer segment. This procedure continues until asuperconductor is achieved with the desired thickness therein.

The present technique may be applied to any of the high temperaturesuperconductors. This includes materials selected from the groupcomprising RBa₂Cu₃O₇₋₈ wherein R is a rare earth, or a Tl-, Pb-, Bi- orHg-based copper-oxide superconductor materials. The oxide materialchosen may be either insulating, conducting or superconducting. However,preferred choices are insulating or superconducting to avoid increasinglosses during use in microwave systems. The oxide material willtypically be well lattice matched to the superconductor material in theplane of the film. The oxide material may be chosen from one of theother high temperature superconducting systems.

Embodiments of the present invention will be described with reference tothe following Examples which are presented for illustrative purposesonly and are not intended to limit the scope of the invention.

EXAMPLE 1

Formation of Thin Films

Several superconductor this film samples were prepared as follows:

Substrate: LaAlO₃, 0.5 millimeters (mm) thick, both sides polished, andpurchased from LITTON-AIRTRON;

Targets: YBa₂Cu₃O₇₋₈ (YBCO): 99.999% purity, purchased fromSUPERCONDUCTIVE COMPONENTS INC.

Ba_(x)Sr_(1-x)TiO₃ (BSTO): 99.999% purity, (produced according to wellknown methods)

The samples were formed using the technique known as “Pulsed LaserDeposition” (A. W. McConnell et al. PHYSICA C225, 7(1994)), using anExcimer Laser, according to the following conditions:

Frequency: 248 nanometers (nm);

Laser Repetition Rate: 2 Hz;

Growth Rate: YBCO: 2.2 Å/sec;

BSTO 2.0 Å/sec;

Vacuum Chamber Base Pressure: 1×10⁻⁶ Torr;

Oxygen pressure during growth: 225 Millitorr (mtorr);

Oxygen flow rate during growth: 2.5 SCCM;

Oxygen pressure after growth: ½ atm;

Growth temperature: 790C.;

LaAlO₃ substrates were attached to the surface of a furnace using aconductive gold paste. The paste was allowed to dry for at least onehour. The furnace was then placed in a growth chamber and then evacuatedto a vacuum level of 1×10⁻⁶ Torr. Once the ‘base pressure’ was achieved,the furnace was heated to the ‘growth’ temperature. Oxygen was thenallowed to flow through the chamber at a rate of 2.5 SCCM achieving apressure of 225 mtorr. Contaminants on the surface of both the YBCO andBSTO targets were removed by allowing the laser to vaporize its surface.A shutter was used to prevent this material from landing on thesubstrate. When this cleaning process was complete, the shutter wasopened and the temperature was allowed to stabilize, and the firstsuperconductor layer segment was grown. The laser was then turned offand the dielectric target was positioned in the laser beam's path. Thedielectric layer was then deposited to form a second layer segment. TheYBCO target was then repositioned in the laser beam's path and the thirdlayer segment was grown. This procedure is not limited to one ‘regrowth’but can be repeated a number of times depending on the number of layersegments required.

Once the growth process was complete, the chamber was filled to apressure of ½ amosphere and the furnace was turned off. The sample coolsto room temperature over a period of two hours.

The following samples were produced:

1a) YBCO 3000 Å/BSTO 300 Å/YBCO 2000 Å/LaAlO₃ Substrate;

1b) 5000 Å YBCO/LaAlO₃ Substrate;

2a) YBCO 3000 Å/BSTO 300 Å/YBCO 3000 Å/BSYO 300 Å/. . . YBCO 2000Å/LaAlO₃ Substrate;

2b) 8000 Å YBCO;

The above four thin films are illustrated in the FIGS. 1 through 6.

X-ray analysis indicates that under these grown conditions, the YBCOlayer segment grows in an (001) orientation (c-axis perpendicular to thesubstrate) and that the BSTO layer segment grows in a (100) orientation.These are the orientations needed for an epitaxial relationship betweenthe substrate, superconductor and dielectric. The data herein indicatesthat the second and subsequent layer segments of superconductor ordielectric maintain an epitaxial relationship.

Comparative resistivity measurements were conducted on composite filmsset out in 1 a, 2 a and the convention YBCO films in 1 b, 2 b. Theseexperiments were carried out using the well known van der Pauw technique(J. Van Der Pauw, PHILLIPS RES. REP. 13, 1 (1958)). These measurementsindicate that the presence of the dielectric material interstices lowersthe critical temperature slightly, by less than 2 degrees. The absolutevalue of the resistance measured corresponds to the total cumulativethickness of the composite films, which is believed to indicate that thedifferent YBCO layer segments are electrically connected through thefilling in of pinholes in the BSTO layer segments. It has also beennoted that the density of pinholes in the BSTO layer segments can bevaried via modification of the growth temperature.

The critical current for samples 2 a and 2 b described above wasmeasured using a non-contact inductive technique as set out in Claussenet al. REV. SCI. INST. 62 (1991)996. Because of the thickness of thefilms, the measuring apparatus used herein was not able to inducecurrents approaching the critical current at 77K for theses films.However at higher temperatures the critical current is reduced, fallingto zero at the critical temperature for the superconductor. The criticalcurrent densities for the two films at 77K were extrapolated bymeasurements taken at a variety of temperatures as shown in FIG. 7. Forthe composite film, 2 a, the critical current density at 77K wasestimated to be approximately 3×10⁶ Amperes per square cm more than afactor of 2 greater than the value obtained for 2 b.

This improvement in the critical current density for the composite filmcompared to the conventional film is attributed to the improvedsmoothness of the composite film. Atomic force microscopy measurementsover a typical 10 micron strip of the film 2 a indicated a film withaverage roughness of 98 Angstroms and maximum deviations of 1100Angstroms. Remarkably, it appears possible to continue this process togrow very thick films. For example, a thick film has been grown usingthe techniques described herein with a cumulative thickness of 2 micronswhich has an average roughness of 140 Angstroms and maximum deviationsof 1100 Angstroms.

These results demonstrate that films several microns in thickness areachievable with average roughnesses of less than 200 Angstroms. Thesefilms have an epitaxial relationship to the substrate and currentdensities comparable to high quality thin films. To illustrate, a 1 cmstrip of such a film with a thickness of, say, 10 microns should intheory be capable of conducting about 3000 Amperes without significantdissipation at a temperature of 77 K.

The term ‘Average Roughness’ means the average value of the absolutedeviation of a surface from a perfectly flat surface. The maximumdeviations were determined by difference in height between the lowestvalley and the highest peak over a typical 10 micron strip on thesurface.

What is claimed is:
 1. A method of forming a single compositesuperconducting film, comprising the steps of: providing a substrate;exposing said substrate to a first atmosphere, including precursors toform a first epitaxial layer segment, exposing said first layer segmentto a second atmosphere, including precursors to form a second epitaxiallayer segment, and exposing said second layer segment to a thirdatmosphere including precursors to form a third epitaxial layer segment,wherein each of said first and third layer segments are each formed froma superconductor material and said second layer segment is formed from amaterial different from said first and third layer segments, whereinsaid first, second and third layer segments have a collective thickness,said third layer segment having an outer surface with a roughness whichis less than that of a single layer of said superconductor material witha thickness equal to said collective thickness; and wherein the secondlayer segment is configured to form a physical and electrical connectionbetween the first and third layers, so that said first, second and thirdlayers collectively form a single superconductive channel.
 2. A methodas defined in claim 1 wherein said second layer segment isdiscontinuous.
 3. A method as defined in claim 1 wherein said first andthird layer segments are formed from the same or different oxidesuperconductor material.
 4. A method as defined in claim 3 wherein saidfirst, second and third layer segments have a collective current densitywhich is substantially equal to the current density of said first layersegment.
 5. A method as defined in claim 3 wherein said oxidesuperconductor material is a high temperature superconductor.
 6. Amethod as defined in claim 5 wherein said superconductor is acopper-oxide superconductor.
 7. A method as defined in claim 2 whereinsaid second layer segment is formed from an oxide material.
 8. A methodas defined in claim 7 wherein said oxide material is an insulatormaterial or a superconductor material.
 9. A method as defined in claim 8wherein said insulator material is a dielectric material selected from agroup comprising SrTiO₃, LaGaO₃, PrGaO₃, NdGaO₃, SrLaGaO₄, CeO₂, LaAlO₃,LaSrAlO₄.
 10. A method as defined in claim 1 wherein said superconductormaterial is selected from the group comprising RBa₂Cu₃O₇₋₈ wherein R isa rare earth, or a Tl-, Pb-, Bi- or Hg-based copper-oxide superconductormaterials.
 11. A method of forming a single composite superconductingfilm, comprising the steps of: providing a substrate; forming a firstepitaxial layer segment on the substrate, and then a second epitaxiallayer on the first layer, and then a third epitaxial layer on the secondlayer, wherein each of said first and third layer segments are eachformed from a superconductor material and said second layer segment isformed from a material different from said first and third layersegments and provides and electrical connection between the first andthird layer segments thereby to form a single composite superconductivechannel which is capable of current densities up to about 3 MA/cm², andwherein said first, second and third layer segments have a collectivethickness, said third layer segment having an outer surface with aroughness which is less than that of a single layer of saidsuperconductor material with a thickness equal to said collectivethickness.
 12. A method as defined in claim 11 wherein said second layersegment is discontinuous.
 13. A method as defined in claim 11 whereinsaid first and third layer segments are formed from the same ordifferent oxide superconductor material.
 14. A method as defined inclaim 13 wherein said first, second and third layer segments have acollective current density which is substantially equal to the currentdensity of said first layer segment.
 15. A method as defined in claim 13wherein said oxide superconductor material is a high temperaturesuperconductor.
 16. A method as defined in claim 15 wherein saidsuperconductor is a copper-oxide superconductor.
 17. A method as definedin claim 12 wherein said second layer segment is formed from an oxidematerial.
 18. A method as defined in claim 17 wherein said oxidematerial is an insulator material or a superconductor material.
 19. Amethod as defined in claim 18 wherein said insulator material is adielectric material selected from a group comprising SrTiO₃, LaGaO₃,PrGaO₃, NdGaO₃, SrLaGaO₄, CeO₂, LaAlO₃, LaSrAlO₄.
 20. A method asdefined in claim 11 wherein said superconductor material is selectedfrom the group comprising RBa₂Cu₃O₇₋₈ wherein R is a rare earth, or aTl-, Pb-, Bi- or Hg-based copper-oxide superconductor materials.
 21. Amethod of forming a single composite superconducting film by separatinga pair of epitaxial superconductive layer segments by an intermediateepitaxial layer segment, wherein the intermediate layer segment isformed from a material different from the pair of layer segments and theintermediate layer segment provided a physical and electrical connectionbetween the pair of layer segments, to form a single compositesuperconductive channel, and wherein the pair of layer segments and theintermediate layer segments have a collective thickness and the film hasan outer surface whose roughness is less than that of a single layer ofsuperconductor material with a thickness equal to said collectivethickness.
 22. A method of forming a single composite superconductingfilm by separating a pair of epitaxial superconductive layer segments byan intermediate epitaxial layer segment, wherein the intermediate layersegment is formed from a material different from the pair of layersegments and the intermediate layer segment and provides an electricalconnection between the paid of layer segments, to form a singlecomposite superconductive channel which is capable of current densitiesup to about 3 MA/cm², and wherein the pair of layer segments and theintermediate layer segments have a collective thickness and the film hasan outer surface whose roughness is less than that of a single layer ofsuperconductor material with a thickness equal to said collectivethickness.
 23. A method of forming a single composite superconductingfilm by separating a pair of epitaxial superconductive layer segments byan intermediate epitaxial layer segment, wherein the intermediate layersegment is formed from a material different from the pair of layersegments and wherein the intermediate layer segment is configured toform a physical and electrical connection between the pair of layersegments, so that said pair of layer segments and said intermediatelayer segment collectively form a single superconductive channel, andwherein the pair of layer segments and the intermediate layer segmentshave a collective thickness and the film has an outer surface whoseroughness is less than that of a single layer of superconductor materialwith a thickness equal to said collective thickness.
 24. A method offorming a single composite superconducting film comprising the steps offorming a first epitaxial layer segment, forming a second epitaxiallayer segment on the first layer segment and forming a third epitaxiallayer segment on the second layer segment, wherein the second layersegment is formed from a material different from the pair of layersegments and wherein the second layer segment is configured to short outthe first and third layer segments, whereby the first, second and thirdlayer segments may be arranged to form together a signal crystal layerwhich is capable of current densities up to about 3 MA/cm², with asingle current carrying channel, and wherein the first, second and thirdhave a collective thickness and the film has an outer surface whoseroughness is less than that of a single layer of superconductor materialwith a thickness equal to said collective thickness.